Sound recording system



Ot- 4, 1932. J. R. BALsLEY SOUND RECORDING SYSTEM Filed Nov. 18, 1929 fzz/fzzar James lalslety.

-- '0nd,' that the discharge `proper density on the film.

Patented ct. 4, 1932 STATES PATENT orion i J'LMES R. BALSLEY, OF BEVERLY HILLS, CALIFORNIA, ASSIGNOR TO FOX FILM CORPO- yBATOJT, OIE* EIOLLYWOOD, CALIFORNIA., A. CORPORATION 0F NEW YORK sonni) nnconnine sYsTnM rninpncauen fried november is, 1929. ser-iai no. 407,924.

This invention has to do With the photographic recording of sound;y and hasffor a l.general object the provision of a sound recording system characterized by high light .efficiency and faithfulness of recordation.

' `According to the present invention the film is light-aected for the purpose of recording, by` a corona discharge from an electrode Y charged by an audio-frequency modulated os- 10. cillating current. The corona discharging 'electrode may be formed, for instance, With a thin discharge'edge mounted parallel to and in alinement with a suitable light slit before Which the film is moved. The electrode may i beincluded in a circuit inductively coupled LEto an 'oscillation circuit carrying the modulated oscillating current, the ratio of coupling being such that a sufficiently high potential to permit corona discharge is provided for the electrode. f

Among the advantages of this system may be mentioned, first, that the glow of a corona discharge is .very actinic in character, and secelectrode may be lo- "Zrcated very .much closer to the film than may the usual present used glow lamp. Both of 'l the above factors contribute toward increased density in the sound record for a given amplified microphone current andv a given light slit. It is Well `known that the accuracy of reproduction is increased as the Width of the light Vslit is decreased, but that the narrovvness of the slit is limited by the lowest light intensity Which will give a suitablev density on'the film.

According to the present invention 1t is possible to'decrease the Widthof the light slit for accuracy of reproduction, While 'thehighly actinic character of D The invention Will be best understood from thefollovving detailed description of a present preferred embodiment thereof, reference for this being had to the accompanying drawing, in Which: l y 5 Fig. l is a diagram of typical apparatus and electrical circuits embodying my invention; f

Fig. 2 is a view taken on line 2 2 of Fig. 1

i and to Fig. 3 is a view taken on line 3-3 of Fig. 1.

the sound impressed thereon. Thesewaves the corona glovv insures Referring now 4to the drawing, Fig. 1 shows the electrical circuit preferablyV embodied in my recording system. The microphone M produces electrical'vvaves corresponding to are transmitted to the amplifier 1(1), conven- '55 tionally illustrated vas being of the vacuuml tube type but which may be of any type capable amplifying Waves Without substantial distortion. The output of amplifier `10 is connected to the modulator-oscillator circuit, '60 generally indicated by the numeral 12, Which may be ofthe Well known Heising circuit type. v

rhisl circuit includes an oscillator tube 20 l and a modulator tube 21, the respective plates l?, and Pm of Which Vare both supplied with direct current from a battery B through a low-'frequency choke coil D, which coil tends to maintain a direct current of constant magl nitude through the two plate circuits in parallel. Vhen the microphone is not in action the potential difference across coil D and battery B is constant, and the tube 20 and its oscillation circuit L1@ generate a high-frequency oscillating `current of constant amplitude. When, however, the microphone M is being acted upon by a'sound Wave, an audio-frequency electromotive force is impressed upon the grid circuit of the modulator, which causes an audio-frequency current to fiow in the plate circuit thereof. Conse- Vquently the direct current supply to the plate circuit of the oscillator tube, and hence yits electro-motive force and the amplitude ofV the hi gh-frequency current fiovving therein, 1s varied or 'modulated in accordance with the impressed audio-frequency wave. l

A coil L2 is inductively coupled to the coil L1 of ycircuit 12, coil L2 having a large num-y ber of turns relative to coil Ll' so that the electromotive force induced therein Will be high. Coils L1 and L2 thus constitute a stepup transformera/ith coil L2 carrying a highktension current. Coil L2 is connected to a 95 Wire 25 which leads to a corona-discharge electrode 26. Electrode 26 has a thin razoredge 27 Which, under the potentials applied thereto, glows With a coronadischarge. Disposed before the electrode razor-edge 2'( and-10o parallel` thereto is av narrow light-slit 28 which may be formed, for instance, by cutting a thin line through a silver coating 29 carried on atransparent base 30. The silver 29, which is connected toy ground, is sufficiently spaced frornthe discharge electrode that under the potentials applied thereto no sparks willjump across the gap, so that a corona discharge only appears at the elec-A trede. kA film F 4on whichthe sound record vis to be made is longitudinally moved transversely to slit 28 and electrode-edge 27 "by means of a sprocket 31 meshing with vfilm perforations 3 Y l 1 `lated high-frequency current of the oscillation ,circuit induces a current in coil L2, and fthe, turnrationlof L1 to L2 being such that a sufficiently high potential lappears `at' the razor-edge v27 ofk electrode 26 tocause a cor- .enadischarge therefrom, the electrode glows -zwithl a light-intensity that varies with the audio-frequency modulations.- `'Ihe .alight from fthis varying corona discharge istransmitted through slit'28 and ,affects the `longivtudfinal'ly moving film-F, producing thereon i a varying density audio-frequency record 35.

y' The liighestpotential for. electr-ode 26, and

the kleast loss in the system, is attained when Y the system is so arranged and balanced that thelefiiecjtive length of coil L2, and wire 25 out to fthe .end of the discharge electrode 26 is an integral multiple of the vfundamental .oscilc fla-ting .frequency wave length of the oscillation circuit. I-n. such a case the reflected wave fram the electrode end of Wire 25 is in phase l with and. aids theforwardly traveling wave, .-aridcauses the potential at thel electrode to reach its highest possiblevaluesy for a given set of conditions. This balance could be realized by `determining the lengt-h of wire 25 in accordance with the fundamental fre- Y zqnency of 'the `oscillation circuit; but it-,isi most -easily .obtained by varying the' funda-l mental ,frequency ofthe oscillation circuit into accordance-with theeffective length of "-icoiljL2 andwire 25 by means vof the variabiley condenser C.v Therefore, in practical Qperatiomlprefer to Vary condenser C until a' voltagelpeakis produced atthe point of; discharge; l v Y rlilies discharge edge 27'of electrode 26 may bearrangedquite close to `the film, and, since a corona discharge is'veryactinic in characteig-a high light intensity and hence aV high .density on the film is obtainable 'even v with the useof a slit of narrower width than i .1h-afs heretofore been possible with any prior system orf-,which I am aware. And since, as

Ais, well .understood 'in the art, the faithfulness of recording is inversely proportional tothe-.width of the slit, within the limits of v piopendensity, on thelilm, v.the Ysystem, is-

Y therefore capable fof., recording -sound Ywith greatly impferedeewfer Y i Ill operation, the' audio-frequency-moda;'l

It will be understoody that thespecii'ic system described herein is merely illustrative of and not restrictive on the invention, and the following claims therefore embrace all such modifications and equivalent arrangei ments asY may fairly be construed to lie within thescope of my invention.

I claim: l i

l. In a sound recording system, the lcoinbination of an electrical vacuum tubeioscil- 'f lator having an oscillation circuit including `an inductance and a capacitance, means for forming a light slit before and p-arallelrto 8 said razor edge ',electrode', said light slit vmeans bei-ng.' includedjiiijsaid high' tension secondary circuit, andthe spacing between the razor edge of the electrode and the light slit means being such that a corona discharge 9 takes place therebetweeiiand Yilluminates the light slit, and means for moving an actinic i nlm before and transversely to said light slit.

2. In a sound recording system, the combination of an electrical vacuum Ytube oscil- ,9i lator' having an voscillation circuit including,

an inductance coiland acondenser, the fundamental oscillation frequency of said 'circuit vbeing' :variable by relative adjustment of its` inductance and capacitance, vmeans 1C Vfor lmodulating the oscillatingcurrent insaid circuit in accordance with an audio frequency input current, `a secondary coil connected to said oscillation circuit and inductively coupled to the inductance coil `of said 10 circuit, a razor edge electrode connected to lthe other Vside of saidsecondary coil, the os- I. cillation frequency of the oscillator being;

adjusted with respect tothe effective length of the electrode circuitjso las to 'give aLA po, 11

tential peakat-sa-id razoredge a grounded-v electrically conductive means forming a vlight slitfbefore andparallel to said razor vedge electrode, the spacing vbetween said razor edge and said -light slit meansv being nl such that a coronay discharge taires: place therebetween 'and illuminates the light slit,

yand vmeans for moving an actinic iilni'before and transversely to said lightV slit.

Y ywitn'esslthat I claim the foregoing I 12( have hereunto subscribed my naine this 17 i day of September 1929.l v

, JAMES R. BALSLEY. 

